Digital communication receivers must sample an analog waveform and then reliably detect the sampled data. Signals arriving at a receiver are typically corrupted by intersymbol interference (ISI), simultaneous switching outputs (SSO), crosstalk, echo, and other noise. For high speed interfaces, such as Double Data Rate (DDR) 3/4, ISI and SSO, for example, have been found to reduce the setup and hold margin of the received data to a data strobe, limiting the potential data rate.
In order to compensate for such channel distortions, communication systems often employ well-known pre-emphasis techniques in the transmitter or equalization techniques in the receiver (or both). On the receiver side, well-known zero equalization or decision-feedback equalization (DFE) techniques (or both) are often employed. Such equalization techniques are widely-used for removing intersymbol interference and other noise. See, for example, R. Gitlin et al., Digital Communication Principles, (Plenum Press, 1992) and E. A. Lee and D. G. Messerschmitt, Digital Communications, (Kluwer Academic Press, 1988), each incorporated by reference herein. Decision-feedback equalization utilizes a discrete-time feedback loop that adds a correction signal, which is a function of previously decoded symbol values, to the channel output.
A communication channel typically exhibits a low pass effect on a transmitted signal. Generally, zero equalization techniques employ a high pass filter to compensate for the low pass effect asserted by the channel. Conventional pre-emphasis techniques attempt to open the received data eye that has been band limited by the low pass channel response. While existing pre-emphasis techniques aim to compensate for channel distortions, they suffer from a number of limitations, which if overcome, could further improve the reliability of data recovery in the presence of channel distortions. For example, existing pre-emphasis techniques, such as changing the drive strength within a given pulse, are limited to serial ISI correction and do not attempt to resolve parallel SSO effects. In addition, existing pre-emphasis techniques within the analog input/output (IO) circuit are difficult to design across the wide range of variables associated with parallel data buses with source synchronous clocking.
A need therefore exists for improved channel compensation techniques that improve the reliability of data detection by a communication receiver.